There were a question and a reply

Over on the ETJ list, Chris Clancy asked about the following sentences:

1. There were a decline and an increase in the respective populations...
2. There was a decline and an increase in the respective populations...

In particular, he wanted to know why someone might mark 1 incorrect in favor of 2.

Existential there is an odd beast which seems to inherit its number from its antecedent except in spoken English where it seems to have recently decided to be a singular pronoun like dummy it, taking 's (e.g., there's two guys standing over there.)

I have no good explanation for what Chris observed, but I have more observations. In a search of Mark Davies's Corpus of Current American English (COCA), the string there were a/an seems to occur almost exclusively in two situations: with irrealis were (e.g., If there were a change...), and followed by quantificational nouns (e.g., there were a couple/dozen/bunch/hundred/awful lot/whole bunch/series etc.). I find but a single obvious instance of coordinated singular nouns following there were a/an: There were a couch and an armchair in what had been the living room, a formulation that strikes me as very odd.

So in answer to Chris's question, I think I would mark the first wrong because English doesn't work that way even though I would expect it to.